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Determining purity of gold

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  • Determining purity of gold

    How do goldsmith determine the purity of gold. They must be able to do this rather quickly and simply. How do they know if there isn't a lead core, or exotic impurity have been added to alter the density.

    I got thinking about this, because composition of steel isn't very simple to determine (or at least not in our shop), but goldsmith don't seem to have this trouble with gold and siver.

    Albert

  • #2
    Albert,

    I'm sure you'll get better replies than mine. But, here goes. You can determine if the sample is pure gold by its density by doing a specific gravity test. This is done with a modified 3-beam balance (gram scale).

    Another way is to immerse the sample in a given volume of water and then measuring the displacement (to finde sample volume). So, if you know the mass (weight for our purposes) and volume, you can calculate the density. Gold is about 19.3 grams/cubic centimeter.

    So, these methods could determine if the sample was pure gold based on the sample density. If your results were appreciably above 19.3 g/cc, than you could surmise that a heavier element(s) had been alloyed with the gold. However,these heavier elements are all in the platinum group on the periodic table and in many cases exceed gold in intrinsic value.

    Platinum is a good example and is alloyed with gold to make one variation of "white gold," but platinum is generally at least twice as expensive as gold. Other metals in this group are palladium, osmium, iridium, etc. Someone wouldn't intentionally alloy gold with these elements without a reason and it wouldn't be for making a "cheap fake gold" to deceive someone.

    If the sample is less than 19.3 g/cc, then you could have any number of metals of lesser value alloyed with the sample to make "fake gold."

    Jewelers also use a "touch stone" and rub a little gold (from the sample) on the stone and then observe its reaction to various acids. I think the stone is either flint or agate.

    Hope this helps a little.

    Regards,

    TIM


    TIM

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    • #3
      One test jewelers use is to compare the sample against test wires of known carat on the touchstone. These test wires are sold in jewelers supply catalogs in sets and are relatively cheap. This however only tests the outer surface of the sample which is rubbed off on the stone.
      The only way to be 100* sure the piece is solid gold would be to cut it open and test the interior.....
      not always possible of course.

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      • #4
        Mark Twain tells how to do a fire assay in his book "Roughing It," in his description of the Comstock silver mines.

        That involves melting the metal, however, so it wouldn't do for testing gold that was already made into something, like jewelry. Twain describes it as the way they determined the content of silver ingots from the mines, which also contained other metals including gold.

        ----------
        Try to make a living, not a killing. -- Utah Phillips
        Don't believe everything you know. -- Bumper sticker
        Everybody is ignorant, only on different subjects. -- Will Rogers
        There are lots of people who mistake their imagination for their memory. - Josh Billings
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        • #5
          Tungsten, which can be found in discarded x-ray tubes, may be the cheapest threat to the identity of gold. It's awfully hard to work, but I suppose a clever scam might be attempted. Tungsten can probably be gold plated after some kind of surface treatment.

          ------------------

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          • #6
            Yeah with things like Nutra-gold casting metal available to blend with real gold this is not as easy as you might think. I remember one of the old scientist in italy making a gold displacement system from measuring the water displaced from sinking a golden crown in water. The goldsmith was hanged if I remember my reading. Da Vinci was the scientist? I am not sure which one.
            A acid test is one method of determining content of gold. Check with pawn shops.. ie: the masters of money in the gold market.

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            • #7
              The acid test is very reliable for testing to see if it is solid gold or gold plated/gold filled. You basically make a scratch in an out of the way place like the inside of a ring and apply some nitric acid. If it turns bright green it is base metal, green in the scratch it is gold over base metal, milky in the scratch is gold over silver. If there isn't a reaction it is all gold baby! I imagine if there was lead in the center it would turn black. Also, Gold dissolves in aqua regia. Check out Tim McCreight's book, "The Complete Metalsmith". The ISBN is 0871922401 It has basic information on all the metals and processes commonly used by jewelers.

              Stuart

              [This message has been edited by hornluv (edited 05-04-2003).]
              Stuart de Haro

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              • #8
                It was Archimedes who thought up the displacement technique, called Archimedes principle(the Eureka! story), as well as the Archimedes screw. He was killed when the Romans took Syracuse, I belive.
                Location: North Central Texas

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                • #9
                  I didn't even know that the Romans had invaded New York.

                  Must have been before Hilary became Senator......

                  Rodg
                  RPease

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                  • #10
                    Albert

                    You can purchase Nuclear devices to determine Alloy content at the flick of a switch (+/- .o1%). I have never seen one for less than $10k used. The most accurate way is mass spectrographic analysis as it can work fom microgram (ppm to ppt accuracy) samples ($250+K US)

                    Since most of us won't be buying the big 'un anytime soon, if you still need to test gold, see a jeweler's supply place (they even have electronic dimond/cubic zirconia testers) - they have everything you need to not get screwed.

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                    • #11
                      Albert,

                      Just came from Ebay and found lots of items for determining gold purity. Prices range from $12 up to over $50 (maybe higher/lower).

                      Don't know how well they work. Probably not as well as Thrud's 1st method, but considerably less costly and you don't have to have the NRC come by every year to check on your radiation levels........


                      Rodg

                      http://cgi.ebay.com/ws/eBayISAPI.dll...4&category=495
                      RPease

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                      • #12
                        Send it all to me.
                        I'll let you know what it is.
                        Tom M.

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